As is well known, an electrical motor is a device whereby electrical power, through passage of current in windings in a “Stator”, is converted into mechanical power by means of rotation of a “Rotor”, through which a fixed mechanical shaft has been inserted. An electrical generator is the reverse of an electrical motor. Mechanical power, through external rotation or drive of a fixed mechanical shaft, inserted in a rotor, is converted into electrical power by creation of current in windings in the stator which may be used to power electrical devices. In the vast majority of cases, the materials used for construction of the stator and rotor are stamped laminations which are stacked into cores to form both the stator and the rotor.
A fundamental principle of Physics for operation of either a motor or a generator is the “Right Hand Rule” whereby a current, rotating in a winding in one direction (x direction), produces an electrical field at 90° to the current (y direction), which, in turn, produces a force in the orthogonal 90° direction (z direction). Thus, the standard design of a motor and a generator is based on a fixed or stationary stator, with electrical windings, which couples with a rotor by means of an electrical field between the stator and the rotor. In the case of a motor, the passage of an electrical current in the stator generates a field which couples with the rotor to produce a force which causes rotation of the rotor and, hence, mechanical power. In the case of a generator, the externally driven rotation of the rotor, together with either an electrical field and/or a current in the rotor, couples with the stator to produce an electrical current in the windings of the stator. The external drive or rotation of the rotor, in and of itself, is not sufficient to start and operate the generator. There also has to be an electrical field and/or a current in the windings of the rotor. The current in the windings of the rotor has to be started in some way. This process is known as “rotor excitation”.
In the market designs for small, portable or stand-by generators, there are several techniques used to achieve an electrical field and/or current in the rotor which can be used to “excite” the rotor (and subsequently generate current in the stator) in the generator:                Permanent magnets may be added to the rotor together with an excitation winding. Rotation of the permanent magnets produces a current in the excitation winding which, in turn, induces a current in the main rotor winding, and, subsequently, an electrical field which produces current in the stator winding.        Flash circuits, together with an excitation winding, may be added to the rotor. Rotation of the rotor initiates a current in the Flash circuit and winding (by means of an applied voltage) which, in turn, induces a current in the main rotor winding, and, subsequently, an electrical field which produces current in the stator winding.        Hard electrical steel, which has the property of retaining a small magnetic field, may be used as part of the design of the rotor whereby, after flashing, the hard electrical steel retains properties of permanent magnetism. Rotation of this permanent magnetic field produces a current in the winding which produces an electrical field to excite or produce current in the stator. There are 2 categories of hard steel used for generator application, being (1) steel that is hardened through alloy content and (2) Full Hard steel achieved through processing. The inherent problem with this design using hard steel is that, while costs are low, compared to the use of magnets or flash circuits, the magnitude of the permanent magnetic field is low. Further, this magnetic field may be reduced in time with high operating temperatures, resulting in a failure of the generator to start.        
The term “full hard” as used in this specification is defined as a condition of cold rolled electrical steel of a coil of the cold rolled electrical steel following sufficient cold rolling, and prior to a subsequent anneal, beyond which there is no further change in hardness or tensile strength of the steel.
A further sophistication of the design of the rotor is that, in some cases, the rotor may be designed in 2 parts, with a main rotor and main rotor winding accompanied by a separate exciter and exciter winding. The alternative simple design is comprised of one rotor construction with both main windings and separate exciter windings.
FIG. 1 shows, in a perspective view, a prior art generator 10 having a main stator 11 surrounding an internal main rotor 12 shown in a fragmentary cut away view within the stator 11. The rotor 12 is mounted on a shaft 13. Also mounted to the shaft 13 is an exciter rotor 14 surrounded by an exciter stator 15 which together with the rotor 14 form an exciter 16. As is well known, slots are provided in both the main rotor and stator and also in the exciter rotor and stator to receive respective windings. As shown in FIG. 1, slots 8 are provided for receiving an exciter stator winding (not shown) and slots 9 are provided for receiving a respective exciter rotor winding (not shown). Laminations for forming the main rotor and stator and the exciter rotor and stator may be stamped to have a same or similar configuration or a different configuration.
A simplified wiring diagram showing the windings for the main rotor and stator and the exciter rotor and stator applicable to generators where the exciter is separate to the main generator is shown in prior art FIG. 2. Here, exciter 16 is schematically illustrated by a dashed box surrounding an excitation source 17 for a flash circuit, or a residual magnetic field (magnets, or hard steel with steel property Hc). FIG. 2 illustrates the exciter stator winding 18, a transfer magnetic field 19 (air gap between stator and rotor), an exciter rotor winding 20, an exciter winding 21 in the main rotor, a transfer magnetic field 22 (rotor electrical steel), a main rotor winding 23, a transfer magnetic field 24 (air gap between rotor and stator), a main stator winding 25, and a power output 26 from the stator. The main rotor is shown in a dashed line box 12 and the main stator is shown in a dashed line box 11.
As indicated, the practice of using Full Hard steel in generator applications has been in use for many years, especially in the USA. However, there is a sophistication which is well known and understood to those skilled in this technology, especially for practices and materials developed in the USA. Industry practice relies on the use of basic Cold Rolled Motor Lamination (CRML) steel, which has chemistries typically described by industry grades such as Type 2, Type 3 or Type 4 (descriptions of properties, but not chemistries, may be found in ASTM A726). Chemistries for these grades encompass ranges of elements described by the following weight percents:
Carbonless than 0.04%Manganese0.10% to 1.5%Phosphorus0.005% to 0.12%Silicon0.10% to 0.60%Aluminum0.05% to 0.35%IronBalance, minus conventional impuritiesfound in normal steelmaking practices
Normal processing of this type of electrical steel for conventional CRML applications in motors follows the sequence:                Melting        Degassing (optional) and alloy addition        Casting        Hot rolling        Pickling        Cold rolling        Annealing (usually box annealing)        Temper rolling        Slitting and stamping into laminations        Final anneal prior to assembly into motors        
This process sequence, coupled with the use of chemistries and grades defined above, produces cold rolled CRML electrical steel with low core loss and excellent permeability, all with competitive cost structures.
The sophistication, well known to those who are expert in this technology, is that the processing of this steel, using the same chemistry or grades, may be changed wherein the step of annealing at the steel mill, following cold rolling, is eliminated. The elimination of this process step results in a full hard grade of CRML steel prior to stamping into laminations. After annealing by the lamination stamper, the core loss values are higher than the core loss for laminations processed conventionally but the permeability is similar for both processing conditions. However, if the laminations are stamped but not annealed, the laminations have high core loss, low permeability but do possess a small amount of residual magnetism which can be measured and defined according to the properties of coercivity (Hc) and retentivity (Br). This is shown in prior art FIG. 3.
In FIG. 3 an example is shown of Hysteresis curves, or the relationship between H (the applied electric field in the winding) and B (the induced magnetic field in the steel) at different inductions (in Tesla). Br is the residual reduced magnetic field (retentivity) where H=0 and Hc is the applied or coerced field (coercivity) required to overcome the retained magnetic field, where B=0.
Thus, it is known to those skilled in the art that the use of one grade of conventional CRML steel, as defined above, may be supplied by a steel mill in a full hard condition and, using one stamping die, may produce both:                Stator laminations, with good core loss and high permeability based on annealing after stamping, and        Rotor laminations, with residual magnetism sufficient for use in excitation of rotor windings in a generator application, based on the absence of annealing after stamping and the absence of annealing following cold rolling at the steel mill.        
The major advantages of the above process and practice are low cost through the use of one stamping die and one material, without the additional costs of permanent magnets or flash circuits. The disadvantages, however, are that the value of residual magnetism is low, subject to possible decay, and the coupling of the rotor with the stator is not as efficient as the use of full electrical steel grades in both rotor and stator (because of the low permeability of full hard grades).
As indicated, there are other conventional alternatives to the use of Full Hard CRML electrical steel involving the use of specifically designed hard steel grades which are described as high residual or high remanence steel grades (refer to TKES and ArcelorMittal literature), especially for rotor designs where there is a separate exciter in addition to the main rotor. The disadvantages of this design approach are that the costs of these steel grades are significantly higher than full hard CRML and, often, there are significant scrap losses/costs since different materials (and stamping dies) may be required for both the stator and the rotor. The further implied disadvantage of hard or high remanence steels, using chemistry to achieve high hardness and, as a result, residual magnetism, is that low core loss and high permeability combinations cannot be achieved (such as in the use of Full Hard CRML steels, with annealing). However, the advantage of hard steels, using chemistry, is that significantly higher coercivity (Hc) may be obtained compared to conventional Full Hard CRML steel grades, as described above.
By convention, the measurement of coercivity (Hc), at a fixed frequency and fixed induction level, is used as a measure of the amount of residual magnetism in different steel grades. As a result, reference to coercivity (Hc) throughout the balance of this disclosure will be used to describe and measure the residual magnetism properties of different grades of steel. The units of A/m (amperes per meter) are used for the measurement of coercivity in this text (noting that conversion to other units, such as oersteds or A/cm, retain the same validity).
Typical properties of a conventional 0.50 mm Full Hard CRML steel in the annealed and not annealed condition are shown in prior art FIG. 4.
FIG. 4 shows typical magnetic properties of 0.50 mm Full Hard CRML steel in the annealed (820° C.) and not annealed conditions according to the prior art.
FIG. 4 clearly shows the major difference or change in properties when comparing properties in the not-annealed and the annealed conditions. Electrical properties in the annealed condition indicate low core loss and excellent permeability, similar to or better than properties of a cold rolled electrical steel, grade 800. Electrical properties in the not-annealed condition are poor in terms of a high core loss and low permeability. However, coercivity Hc or permanent magnetism, is relatively high.
It is noted that, while core loss is dependent on thickness, coercivity is independent of thickness for different steel grades.
A comparison of electrical and magnetic properties for Full Hard CRML and a typical grade of high residual magnetism (high remanence) commercial steel, according to the prior art is shown in FIG. 5.
FIG. 5 shows a comparison of electrical and magnetic properties for Full Hard CRML (0.50 mm) and a typical grade of high residual magnetism (high remanence) commercial steel (1.00 mm) (both representing the prior art).
Two conclusions are immediately obvious from the data in FIG. 5:                1. Coercivity (Hc) of Full Hard conventional CRML steel is significantly lower (worse) than coercivity (Hc) for high remanence commercial steel        2. After anneal core loss and permeability of Full Hard conventional CRML steel is significantly better than after anneal core loss and permeability for high remanence commercial steel, to the extent that this type of steel should not be used where electrical efficiency is important.        
The importance of FIG. 5 is that it clearly shows that a unique combination of electrical properties (low core loss and high permeability in the after anneal condition together with good coercivity in the not-annealed condition) is possible with conventional CRML in the full hard condition but not possible with conventional high remanence hard steel.
To date, it has not been known in the prior art to increase coercivity values for Full Hard CRML steel to values close to those for high residual magnetism other than to resort to the use of high residual magnetism commercial steel. The disadvantages of higher cost and poor magnetic properties remain for these grades (high remanence hard steel).